Because it is obvious that if the 18th is the only Amendment that's ever been repealed, it never had the support that those who pushed for it had claimed. It only lasted 14 years, which isn't that much time in the scheme of things.
Perhaps they should have gone the other route as I've previously mentioned, as IF there was already major opposition to the Amendment at it's proposal, then a simple Act of Congress would have sufficient to prohibit the sale, manufacture, and transportation of alcohol. That would have appeased the proponants of Prohibition, and would have avoided both the process of amending the Consitution, and later the necessity of amending it again only 14 years later to repeal that original amendment.
You still haven't answered the question as to why it was necessary to add a Constitutional Amendment to prohibit the sale, manufacture, and transportation of alcohol when a simple Act of Congress would have been sufficient to do just that.
Wrong, the support in 1919 was overwhelming. Opposition grew over the following 14 years resulting in its eventual repeal.
BTW, it did NOT do that. Read it.